1. Field of the Invention
The present invention relates to a method and to a device intended for land seismic prospecting by simultaneous emission in the ground of seismic signals emitted by several vibrators or groups of vibrators, the signals being obtained by coding a signal by pseudo-random sequences, and notably a periodic signal phase modulated by such sequences.
2. Description of the Prior Art
There are well-known land seismic prospecting methods comprising transmission in the ground, for several seconds, of a periodic signal whose frequency varies continuously within a frequency band, reception by pickups of the signals reflected by underground reflectors and recording of the received signals. As a result of the emission time, the signals picked up are combinations of signals reflected by reflectors arranged at very different depths. The image of the various reflectors in the subsoil can only be recovered by processing the signals picked up, including correlation thereof with the signals emitted. The processing result is identical to that obtained by convolving the autocorrelation function of the signal emitted by the reflection coefficients of the various reflectors. The seismic trace obtained is the image of the interfaces between the various geologic layers midway between the emission and reception points. Such a method is for example described in U.S. Pat. No. 2,688,124.
This method however has some drawbacks. The autocorrelation function that is obtained in this case exhibits secondary peaks on either side of the main peak, whose amplitude is quite significant. Furthermore, a time interval at least equal to the loop travel time of the waves emitted down to the deepest reflector in the zone explored, referred to as xe2x80x9clistening intervalxe2x80x9d, has to be provided between two successive emission sequences so that the strong signals picked up at the beginning of the corresponding recording sequence cannot conceal the weaker signals coming from more distant reflectors, picked up at the end of the previous recording sequence. The necessary emission interruptions during a relatively long listening time interval have the effect of limiting the energy transmitted.
There is also another well-known method wherein various vibrators emit simultaneously signals with an identical frequency sweep, the lag between their emissions being at least equal to the listening time.
Pseudo-random Binary Sequences
Other known land seismic prospecting methods use a technique that is well-known in the field of communications and radars. They comprise using periodic sources that transmit signals obtained by modulating the phase of a periodic carrier signal by a binary signal or pseudo-random code consisting of a sequence of elements that can take on two logical values 0 or 1. As shown in FIG. 1, the order of succession of these values is selected so as to have a random character.
When, for such a code, any sequence of n successive bits (n integer) can be repeated identically only after a sequence of (2nxe2x88x921) bits, the sequence is referred to as xe2x80x9cBinary Sequence of Maximum Lengthxe2x80x9d (BSML). These pseudo-random binary sequences can be used for phase modulation of a periodic signal, each element of the sequence being associated with a period of the signal, by keeping or by inverting the sign according to whether it is a 1 or a 0. The term xe2x80x9celementary sequencexe2x80x9d designates a portion of the periodic signal modulated by a binary sequence of (2nxe2x88x921) terms, of length (2nxe2x88x921)Tc, where Tc is the period of the carrier signal.
Correlation, by an elementary sequence, of a signal consisting of the repetition of this elementary sequence modulated by the elementary sequence (FIG. 3B) gives (FIG. 3C) peaks spaced out (in time) by the length of sequence Ts and a minimum (or even zero) level between the peaks or, more exactly, over the length of the sequence minus the period of the carrier Tc. The ratio of the correlation peak to the correlation noise is equal to the number of terms of the sequence.
Such methods are for example described in U.S. Pat. Nos. 3,234,504, 3,264,606, 4,034,333 and 4,069,470.
U.S. Pat. No. 3,234,504 describes a seismic prospecting method wherein modulated signals without cusps, whose amplitude spectrum is centered on the zero frequency, are applied to a vibrator.
U.S. Pat. No. 4,780,856 filed by the assignee describes a marine seismic prospecting method wherein the emission means comprise at least one vibrator towed by a boat progressing continuously along a seismic profile to be studied, emitting an uninterrupted series of sequences consisting each of a periodic carrier signal phase modulated by a pseudo-random binary coding signal of maximum length. The signals reflected by the discontinuities of the medium are correlated with the coded signals emitted so as to obtain correlation peaks at time intervals shorter than or at most equal to the period of repetition of the successive emission sequences. The periodic source can be the sole source, correlation being carried out between the signals received and alternately two sequences of signals emitted deducible from one another by a time lag shorter than the period of repetition of the sequences. A lag equal to the half-period of repetition of the sequences of signals emitted is for example selected.
It is also possible to use at least two periodic sources emitting simultaneously sequences of identical signals but with a time lag between them, and a correlation is established between the received signals, which correspond to the signals emitted simultaneously by the sources, and at least one sequence of coded signals, so as to alternately obtain correlation peaks corresponding to each periodic source.
The use, in land seismic prospecting, of vibrators emitting simultaneously or insufficiently separated in time has drawbacks linked with various factors: the autocorrelation noise, the harmonics and the slow waves.
Autocorrelation Noise
The vibroseismic signal is compressed by correlating the signals recorded by the signal controlling the vibrator (or by a combination of the signals of the plate and mass accelerometers of the vibrator). The equivalent of the correlation of the series of reflection coefficients is thus obtained by autocorrelation of the emitted signal. The emitted signal is generally a frequency linear sweep whose amplitude spectrum has either a crenellated shape, or it is preferably a bell curve to reduce the amplitude of the bounces.
The question of autocorrelation noises arises for all the vibroseismic records. The bounces decrease with time as a function of 1/t substantially. For an isolated record, the autocorrelation bounces of the large values at the beginning of a trace are sufficiently attenuated when the weakest reflections at the end of a trace reappear. In the case of a frequency slip sweep, recording is semi-continuous and the bounces of the large values are found in front and behind, and they can interfere with the low values of the deep reflections of the previous shot if the slip time is insufficient.
Harmonics
For an isolated vibroseismic record, the harmonic distortion adds oscillations to the correlated signal. If the sweep is carried out from the low frequencies to the high frequencies, the oscillations due to the correlation of the harmonics by the control signal are precursors. Thus, except for the closest traces comprising the surface noise, the noises due to the correlation of the harmonics mix with an earlier, therefore in principle stronger signal. For continuous slip sweep type records, the noise due to the harmonics of the early arrivals of a shot can be superposed on the late and therefore weaker arrivals of the previous shot.
Slow Waves
If the time interval between the start of two successive shots decreases, there is a risk the slowest waves of a shot, air waves and surface waves, may be found on the next shot. The sweeps being identical from one shot to the next, the air wave and the surface noises will be compressed similarly on the two shots.
Minimum Correlation Sequences
It is well-known in the art that there are, for each size of binary sequence of maximum length, minimum correlation pairs for which the ratio of the central peak to the greatest secondary peak is:             2      xe2x80x3        -    1        1    +          2              wholepart        ⁢                  xe2x80x83                ⁢                  (                                    n              +              2                        2                    )                    
i.e. about 30 dB for n=11, corresponding to 2xe2x80x3xe2x88x921=2047 periods of the carrier signal (34 seconds for a 60 Hz carrier). There are connected sequences whose correlations are minimum two by two. The number of sequences of each set depends on the size of the sequences. For a sequence with 2047 elements, this number is 4.
French Patent 2,818,753, filed by the assignee, describes a method of prospecting or monitoring operations in an underground formation by elastic waves. It comprises emission, in the ground, of elementary sequences formed by coding of a signal by pseudo-random sequences, reception and recording of the signals reflected by the subsoil discontinuities in response to the signals emitted, and processing of the signals recorded by correlation with signals formed from the emitted signals.
The signals are emitted simultaneously by several seismic sources each controlled by a signal consisting of an elementary sequence whose length is at least equal to the product of the number of seismic sources vibrating simultaneously by the listening time, the respective contributions of the various seismic sources being separated by correlating the signals received and recorded by a cyclic signal portion constructed from the elementary sequence, extended, on the right and on the left (before and after), with parts whose length is at least equal to the listening time, obtained by circular permutation, this elementary sequence occupying the center of the cyclic signal portion.
The signals can also be emitted by several seismic sources each controlled by a signal consisting of an elementary sequence extended, before and after, with parts at least equal to the listening time, obtained by circular permutation, this elementary sequence occupying the center of this cyclic signal portion, the respective contributions of the various seismic sources being separated by correlating the signals received and recorded by signals constructed from the elementary sequence.
This method, which uses periodic signals obtained by phase modulation of a carrier signal by pseudo-random binary sequences, and the simultaneous recording of several seismic sources, allows minimizing the correlation noises and prevents harmonic correlation noises. It is better suited to simultaneous recording than the frequency sweeps conventionally used in land seismic prospecting.
However, it has been observed that the phase modulation of a sinusoid by a pseudo-random code leads to cusps at code sign changes. The signal is distorted upon emission. This distortion is expressed differently when passing between two like terms of the sequence and between two unlike terms. It follows therefrom that the various terms cannot be correctly summed, so that the correlation does not reach the desired performance.
The method according to the invention allows prospecting or monitoring operations in an underground formation by elastic waves. It comprises simultaneous emission, in the ground, of seismic waves by several seismic sources each controlled by a signal consisting either of an elementary sequence formed by coding of a signal by pseudo-random sequences, the length of the elementary sequence being at least equal to the product of the number of seismic sources vibrating simultaneously by the listening time, or respectively of the elementary sequence extended, before and after, with parts whose length is at least equal to the listening time, reception and recording of the signals reflected by the subsoil discontinuities in response to the signals emitted, and processing of the signals recorded, the respective contribution of each one of the various seismic sources being separated by correlating the received signals and recorded either by signals constructed from the elementary sequence, extended, before and after, with parts whose length is at least equal to the listening time, obtained by circular permutation, or respectively of the elementary sequence. The signals applied to the various vibrators are formed from a periodic signal whose amplitude and derivatives with respect to time are equated to zero at the beginning and at the end of each period, and such that integral of the periodic signal over a period is zero.
The integral over a period being zero prevents the frequency spectrum from being centered on the zero frequency (which of course cannot be emitted in practice) and therefore avoids the real signal deformations resulting therefrom.
According to an embodiment, a signal of frequency f, of the form sin 2xcfx80ft(1xe2x88x92cos 2xcfx80ft), is used as the periodic signal.
With this additional condition applied to the periodic signal, the separation obtained of the contributions of the various seismic sources to the recorded signals is much better.
According to a preferred embodiment, the elementary sequences are formed by phase modulation of the periodic signal and these sequences are applied to vibrators.
The same control sequence is, for example applied to all the vibrators, with a time lag and a circular permutation. The time lags between two sequences are at least equal to the listening time, and the length of the elementary control sequence is at least equal to the sum of the time lags and of the listening time.
According to another embodiment, the vibrators are controlled by a connected set of minimum crosscorrelation sequences. Each vibrator emits a vibration with a sequence independent of the other vibrators.
According to another embodiment, the previous two modes are combined by dividing the vibrators into groups, and each group is assigned a pseudo-random sequence belonging to the same group of minimum crosscorrelation sequences, the various vibrators of the same group being controlled by the same elementary sequence with the time lags.
The device which prospects or monitors in a formation by elastic waves according to the invention comprises at least one group of m vibrators, a control unit which applies to the vibrators elementary sequences formed by phase modulation of a periodic signal by pseudo-random sequences, seismic receivers coupled with the formation, a system which acquires and records the seismic signals reflected by the subsoil discontinuities in response to the periodic emitted signals and a system which processes the recorded seismic signals, by correlation with part of the periodic emitted signals. The control unit comprises a periodic signal generator and a modulation set for forming elementary sequences phase modulated by a pseudo-random sequence, and connection means for applying simultaneously to the various vibrators of each group the signals produced by the modulation set.
According to a first embodiment, the modulation set comprises means for generating at least one pseudo-random control sequence, time lag means for forming at least one set of m elementary sequences with time lags between each one of the elementary sequences respectively, means which phase modulates the periodic signals generated by the generator respectively by the m elementary sequences, generating m phase modulated periodic signals.
According to another embodiment, the modulation set comprises means for generating at least one pseudo-random control sequence, modulation means for modulating, by the sequence, the periodic signals generated by the generator, and time lag means for producing m modulated periodic signals with a time lag between each signal.
The device comprises, for example, p groups of vibrators, and the control unit generates p minimum crosscorrelation elementary sequences and to apply these sequences with a time lag to the vibrators of each group.
According to an embodiment of the device, the vibrators are installed permanently for seismic monitoring of an underground zone.